Many different types of lasers have been developed in recent years. These different types of lasers have many different characteristics and each are suitable for particular special applications. Many of the lasers because of their peculiar characteristics are more adaptable to certain applications than others.
Certain applications such as communications, require the ability to modulate the laser. The modulation may be either amplitude modulation (AM) or frequency modulation (FM). Frequency modulation of the carrier wave is preferred in communication systems because of its better signal to noise ratio as compared to amplitude modulation.
Injection lasers are one type of laser that is suitable for communication and other similar systems. These lasers are typically modulated by a modulation of the laser's bias current. U.S. Pat. No. 4,485,474 illustrates a laser provided with a closely coupled external tunable resonator for high frequency modulation of the laser's modes. The resonator includes a crystal and a bias contact secured to the top of the crystal resonator and to a source of bias voltage or modulating voltage. The bias voltage or modulating voltage on the crystal modifies the optical properties (changes the index of refraction) of the crystal and thereby provides a tunable but low Q (low coherence) resonator system allowing a multiplicity of laser frequencies.
U.S. Pat. No. 4,101,845 discloses a method for producing coherent mode pure radiation modulated at a high bit rate, having two optically coupled injection lasers which may be independently controlled. A direct current is passed through a first injection laser having good optical quality and good spectral purity of its initial radiation, the direct current being such that said first injection laser is biased above the threshold value, and biasing a further injection laser having a low optical quality in comparison to the first injection laser, by a direct current passing through it and controlling said further injection laser additionally by a modulation current superimposed on this direct current.
It is also well known that the performance of short cavity lasers such as injection lasers can be improved by the addition of passive cavity or grating structures. Such devices may be added externally or integrated with the laser on the same substrate. For example, see LASER FOCUS/ELECTRO-OPTICS of October, 1988, page 112 which describes distributed feedback diode lasers with single-longitudinal mode output that include an external cavity defined by a fiber coupled to the laser cavity or by an integral waveguide fabricated directly onto the laser substrate.
The purpose of such short cavity lasers is usually to narrow the laser's linewidth and to stabilize the laser output frequency. The major disadvantage of the known stabilization techniques is the nontunability of such systems. For frequency division multiplexing (FDM) applications rapid electronic tuning of the laser's frequency is necessary. Accordingly, it is believed that the shortcomings of prior systems is overcome by the arrangement and method of the present invention which provides an electronically tunable system while maintaining the advantages of a narrow source linewidth and carrier frequency stability.